Materials and structures for enhancing the performance of organic light emitting devices

ABSTRACT

A device is provided, having an anode, a cathode, and a first organic layer disposed between the anode and the cathode. The first organic layer comprises a material that produces phosphorescent emission when a voltage is applied between the anode and the cathode. A second organic layer is disposed between the first organic layer and the cathode. The second organic layer is in direct contact with the first organic layer. The second organic layer may comprise an aromatic hydrocarbon material, comprising an aromatic non-heterocyclic hydrocarbon core optionally substituted, and wherein the substituents are the same or different, and each is selected from the group consisting of alkyl, alkenyl, alkynyl, aryl, heteroalkyl, substituted aryl, substituted heteroaryl and heterocyclic groups. The second organic layer may comprise a material having a molecular dipole moment less than about 2.0 debyes, such that the device has an unmodified external quantum efficiency of at least about 3% and a lifetime of at least about 1000 hours at an initial luminance of about 100 to about 1000 cd/m 2 . The second organic layer may be in direct contact with the cathode, or there may be a separate organic layer between the second organic layer and the cathode.

This application is a continuation-in-part of U.S. application Ser. No.10/626,579, filed Jul. 25, 2003, and which is incorporated by referencein its entirety.

FIELD OF THE INVENTION

The present invention relates to efficient organic light emittingdevices (OLEDs), and more specifically to phosphorescent aromaticorganic non-heterocyclic hydrocarbon materials with improved stabilityand efficiency used in such devices.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

OLED devices are generally (but not always) intended to emit lightthrough at least one of the electrodes, and one or more transparentelectrodes may be useful in an organic opto-electronic device. Forexample, a transparent electrode material, such as indium tin oxide(ITO), may be used as the bottom electrode. A transparent top electrode,such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which areincorporated by reference in their entireties, may also be used. For adevice intended to emit light only through the bottom electrode, the topelectrode does not need to be transparent, and may be comprised of athick and reflective metal layer having a high electrical conductivity.Similarly, for a device intended to emit light only through the topelectrode, the bottom electrode may be opaque and/or reflective. Wherean electrode does not need to be transparent, using a thicker layer mayprovide better conductivity, and using a reflective electrode mayincrease the amount of light emitted through the other electrode, byreflecting light back towards the transparent electrode. Fullytransparent devices may also be fabricated, where both electrodes aretransparent. Side emitting OLEDs may also be fabricated, and one or bothelectrodes may be opaque or reflective in such devices.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” a second layer, the firstlayer is disposed further away from substrate. There may be other layersbetween the first and second layer, unless it is specified that thefirst layer is “in physical contact with” the second layer. For example,a cathode may be described as “disposed over” an anode, even thoughthere are various organic layers in between.

SUMMARY OF THE INVENTION

A device is provided, having an anode, a cathode, and a first organiclayer disposed between the anode and the cathode. The first organiclayer comprises a material that produces phosphorescent emission when avoltage is applied between the anode and the cathode. A second organiclayer disposed between the first organic layer and the cathode is alsoprovided. The second organic layer is in direct contact with the firstorganic layer. The second organic layer may comprise an aromaticnon-heterocyclic hydrocarbon compound. Particularly suitable aromatichydrocarbon materials include materials having the following structures:

Each aromatic ring may optionally be substituted. Particularly suitablearomatic non-heterocyclic hydrocarbon materials with substituted ringsinclude materials having the following structures:

wherein:

-   -   R₁—R₁₀ each represent no substitution, mono-, di-, or tri-        substitution, and wherein R₁—R₁₀ are the same or different        substituents, and each is selected from the group consisting of        alkyl, alkenyl, aryl, heteroalkyl and substituted aryl groups;    -   R₁₁—R₁₃, R₁₅—R₁₈, R₂₉—R₄₅ each represents no substitution,        mono-, di-, or tri- substitution, and wherein R₁₁—R₁₃, R₁₅—R₁₈,        R₂₉—R₄₅ are the same or different substituents, and each is        selected from the group consisting of alkyl, alkenyl, alkynyl,        aryl, heteroalkyl, and substituted aryl;    -   R₁₄, R₁₉, and R₂₀—R₂₈ each represents no substitution, mono-,        di-, or tri- or tetra- substitution and R₁₄, R₁₉, and R₂₀—R₂₈        are the same or different substituents, and each is selected        from the group consisting of alkyl, alkenyl, alkynyl, aryl,        heteroalkyl and substituted aryl, and    -   substituent R of each hydrocarbon structure may be linked        together to form cyclic substituents such as cycloalkyl or        aromatic non-heterocyclic rings.

The second organic layer may comprise a material having a moleculardipole moment less than about 2.0 debyes, such that the device has anunmodified external quantum efficiency of at least about 3%, and alifetime of at least about 1000 hours at an initial luminance of about100 to about 1000 cd/m². The second organic layer may be in directcontact with the cathode, or there may be a separate organic layerbetween the second organic layer and the cathode. Other aromatichydrocarbon materials may be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device having separate electrontransport, hole transport, and emissive layers, as well as other layers.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 shows a device having an organic enhancement layer that is not indirect contact with the cathode.

FIG. 4 shows current-voltage characteristics for examples 8-10 andcomparative example 11.

FIG. 5 shows luminous efficiency vs. brightness for examples 8-10 andcomparative example 11.

FIG. 6 shows external quantum efficiency vs. current density forexamples 8-10 and comparative example 11.

FIG. 7 shows normalized electroluminescence spectra for examples 8-10using HPT as the ETL2.

FIG. 8 operational stability for examples 8-10 using HPT as the ETL2 atroom temperature under constant direct current of 40 mA/cm² and atinitial luminance (L₀) of 955 nits.

FIG. 9 shows the operational stability for examples 8-10 using HPT asthe ETL2 at 60° C. under constant direct current at L₀=955 cd/m².

FIG. 10 shows the current-voltage characteristics for example 12.

FIG. 11 shows luminous efficiency vs. brightness for example 12.

FIG. 12 and external quantum efficiency vs. current density for example12.

FIG. 13 shows the current-voltage characteristics for examples 13 andcomparative example 14.

FIG. 14 luminous efficiency vs. brightness for example 13 andcomparative example 14.

FIG. 15 external quantum efficiency vs. current density for example 13and comparative example 14 using HPT as the ETL2.

FIG. 16 shows the normalized electroluminescence spectra of example 13using HPT as the ETL2.

FIG. 17 shows operational stability for examples 13 using HPT as theETL2 at room temperature under constant direct current of 40 mA/cm².

FIG. 18 shows the current-voltage characteristics for examples 15-17.

FIG. 19 shows luminous efficiency vs. brightness for examples 15-17.

FIG. 20 shows external quantum efficiency vs. current density forexamples 15-17.

FIG. 21 shows the normalized electroluminescence spectra for example15-17 using TSBF as the ETL2.

FIG. 22 shows the operational stability of examples 15-17 using TSBF asthe ETL2 at room temperature under constant direct current of 40 mA/cm².

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence may be referred to asa “forbidden” transition because the transition requires a change inspin states, and quantum mechanics indicates that such a transition isnot favored. As a result, phosphorescence generally occurs in a timeframe exceeding at least 10 nanoseconds, and typically greater than 100nanoseconds. If the natural radiative lifetime of phosphorescence is toolong, triplets may decay by a non-radiative mechanism, such that nolight is emitted. Organic phosphorescence is also often observed inmolecules containing heteroatoms with unshared pairs of electrons atvery low temperatures. 2,2′-bipyridine is such a molecule. Non-radiativedecay mechanisms are typically temperature dependent, such that amaterial that exhibits phosphorescence at liquid nitrogen temperaturesmay not exhibit phosphorescence at room temperature. But, asdemonstrated by Baldo, this problem may be addressed by selectingphosphorescent compounds that do phosphoresce at room temperature.Representative emissive layers include doped or un-doped phosphorescentorgano-metallic materials such as disclosed in U.S. Pat. Nos. 6,303,238and 6,310,360; U.S. patent application Publication Nos. 2002-0034656;2002-0182441; amd 2003-0072964; and WO-02/074015.

Generally, the excitons in an OLED are believed to be created in a ratioof about 3:1, i.e., approximately 75% triplets and 25% singlets. See,Adachi et al., “Nearly 100% Internal Phosphorescent Efficiency In AnOrganic Light Emitting Device,” J. Appl. Phys., 90, 5048 (2001), whichis incorporated by reference in its entirety. In many cases, singletexcitons may readily transfer their energy to triplet excited states via“intersystem crossing,” whereas triplet excitons may not readilytransfer their energy to singlet excited states. As a result, 100%internal quantum efficiency is theoretically possible withphosphorescent OLEDs. In a fluorescent device, the energy of tripletexcitons is generally lost to radiationless decay processes that heat-upthe device, resulting in much lower internal quantum efficiencies. OLEDsutilizing phosphorescent materials that emit from triplet excited statesare disclosed, for example, in U.S. Pat. No. 6,303,238, which isincorporated by reference in its entirety.

Phosphorescence may be preceded by a transition from a triplet excitedstate to an intermediate non-triplet state from which the emissive decayoccurs. For example, organic molecules coordinated to lanthanideelements often phosphoresce from excited states localized on thelanthanide metal. However, such materials do not phosphoresce directlyfrom a triplet excited state but instead emit from an atomic excitedstate centered on the lanthanide metal ion. The europium diketonatecomplexes illustrate one group of these types of species.

Phosphorescence from triplets can be enhanced over fluorescence byconfining, preferably through bonding, the organic molecule in closeproximity to an atom of high atomic number. This phenomenon, called theheavy atom effect, is created by a mechanism known as spin-orbitcoupling. Such a phosphorescent transition may be observed from anexcited metal-to-ligand charge transfer (MLCT) state of anorganometallic molecule such as tris(2-phenylpyridine)iridium(III).

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, and a cathode 160. Cathode 160 is acompound cathode having a first conductive layer 162 and a secondconductive layer 164. Device 100 may be fabricated by depositing thelayers described, in order.

Substrate 110 may be any suitable substrate that provides desiredstructural properties. Substrate 110 may be flexible or rigid. Substrate110 may be transparent, translucent or opaque. Plastic and glass areexamples of preferred rigid substrate materials. Plastic and metal foilsare examples of preferred flexible substrate materials. Substrate 110may be a semiconductor material in order to facilitate the fabricationof circuitry. For example, substrate 110 may be a silicon wafer uponwhich circuits are fabricated, capable of controlling OLEDs subsequentlydeposited on the substrate. Other substrates may be used. The materialand thickness of substrate 110 may be chosen to obtain desiredstructural and optical properties.

Anode 115 may be any suitable anode that is sufficiently conductive totransport holes to the organic layers. The material of anode 115preferably has a work function higher than about 4 eV (a “high workfunction material”). Preferred anode materials include conductive metaloxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO),aluminum zinc oxide (AlZnO), and metals. Anode 115 (and substrate 110)may be sufficiently transparent to create a bottom-emitting device. Apreferred transparent substrate and anode combination is commerciallyavailable ITO (anode) deposited on glass or plastic (substrate). Aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. No. 5,844,363, which is incorporated by reference in itsentirety. Anode 115 may be opaque and/or reflective. A reflective anode115 may be preferred for some top-emitting devices, to increase theamount of light emitted from the top of the device. The material andthickness of anode 115 may be chosen to obtain desired conductive andoptical properties. Where anode 115 is transparent, there may be a rangeof thickness for a particular material that is thick enough to providethe desired conductivity, yet thin enough to provide the desired degreeof transparency. Other anode materials and structures may be used.

Hole transport layer 125 may include a material capable of transportingholes. Hole transport layer 125 may be intrinsic (undoped), or doped.Doping may be used to enhance conductivity. α-NPD and TPD are examplesof intrinsic hole transport layers. An example of a p-doped holetransport layer is m-MTDATA doped with F₄-TCNQ at a molar ratio of 50:1,as disclosed in U.S. patent application Ser. No. 10/173,682 to Forrestet al., which is incorporated by reference in its entirety. Other holetransport layer materials and structures may be used.

Emissive layer 135 may include an organic material capable of emittinglight when a current is passed between anode 115 and cathode 160.Preferably, emissive layer 135 contains a phosphorescent emissivematerial, although fluorescent emissive materials may also be used.Phosphorescent materials are preferred because of the higher luminescentefficiencies associated with such materials. Emissive layer 135 may alsocomprise a host material capable of transporting electrons and/or holes,doped with an emissive material that may trap electrons, holes, and/orexcitons, such that excitons relax from the emissive material via aphotoemissive mechanism. Emissive layer 135 may comprise a singlematerial that combines transport and emissive properties. Whether theemissive material is a dopant or a major constituent, emissive layer 135may comprise other materials, such as dopants that tune the emission ofthe emissive material. Emissive layer 135 may include a plurality ofemissive materials capable of, in combination, emitting a desiredspectrum of light. Examples of phosphorescent emissive materials includeIr(ppy)₃. Examples of fluorescent emissive materials include DCM andDMQA. Examples of host materials include Alq₃, CBP and mCP. Examples ofemissive and host materials are disclosed in U.S. Pat. No. 6,303,238 toThompson et al., which is incorporated by reference in its entirety.Emissive material may be included in emissive layer 135 in a number ofways. For example, an emissive small molecule may be incorporated into apolymer. Other emissive layer materials and structures may be used.

Electron transport layer 140 may include a material capable oftransporting electrons. Electron transport layer 140 may be intrinsic(undoped), or doped. Doping may be used to enhance conductivity. Alq₃ isan example of an intrinsic electron transport layer material. An exampleof an n-doped electron transport layer material is BPhen doped with Liat a molar ratio of 1:1, as disclosed in U.S. patent application Ser.No. 10/173,682 to Forrest et al., which is incorporated by reference inits entirety. Other electron transport layer materials and structuresmay be used.

The charge carrying component of the electron transport layer may beselected such that electrons can be efficiently injected from thecathode into the LUMO (Lowest Unoccupied Molecular Orbital) level of theelectron transport layer. The “charge carrying component” is thematerial responsible for the LUMO that actually transports electrons.This component may be the base material, or it may be a dopant. The LUMOlevel of an organic material may be generally characterized by theelectron affinity of that material while relative electron injectionefficiency of a cathode may be generally characterized in terms of thework function of the cathode material. This means that the preferredproperties of an electron transport layer and the adjacent cathode maybe specified in terms of the electron affinity of the charge carryingcomponent of the ETL and the work function of the cathode material. Inparticular, to achieve high electron injection efficiency, the workfunction of the cathode material is preferably not greater than theelectron affinity of the charge carrying component of the electrontransport layer by more than about 0.75 eV, more preferably, by not morethan about 0.5 eV. Most preferably, the electron affinity of the chargecarrying component of the electron transport layer is greater than thework function of the cathode material. Similar considerations apply toany layer into which electrons are being injected.

Cathode 160 may be any suitable material or combination of materialsknown to the art, such that cathode 160 is capable of conductingelectrons and injecting them into the organic layers of device 100.Cathode 160 may be transparent or opaque, and may be reflective. Metalsand metal oxides are examples of suitable cathode materials. Cathode 160may be a single layer, or may have a compound structure. FIG. 1 shows acompound cathode 160 having a thin metal layer 162 and a thickerconductive metal oxide layer 164. In a compound cathode, preferredmaterials for the thicker layer 164 include ITO, IZO, and othermaterials known to the art. U.S. Pat. Nos. 5,703,436 and 5,707,745,which are incorporated by reference in their entireties, discloseexamples of cathodes including compound cathodes having a thin layer ofmetal such as Mg:Ag with an overlying transparent,electrically-conductive, sputter-deposited ITO layer. The part ofcathode 160 that is in contact with the underlying organic layer,whether it is a single layer cathode 160, the thin metal layer 162 of acompound cathode, or some other part, is preferably made of a materialhaving a work function lower than about 4 eV (a “low work functionmaterial”). Other cathode materials and structures may be used.

Blocking layers may be used to reduce the number of charge carriers(electrons or holes) and/or excitons that leave the emissive layer. Anelectron blocking layer 130 may be disposed between emissive layer 135and the hole transport layer 125, to block electrons from leavingemissive layer 135 in the direction of hole transport layer 125.Similarly, a hole blocking layer 140 may be disposed between emissivelayer 135 and electron transport layer 145, to block holes from leavingemissive layer 135 in the direction of electron transport layer 140.Blocking layers may also be used to block excitons from diffusing out ofthe emissive layer. The theory and use of blocking layers is describedin more detail in U.S. Pat. No. 6,097,147 and U.S. patent applicationSer. No. 10/173,682 to Forrest et al., which are incorporated byreference in their entireties. The conventional “blocking layer” isgenerally believed to reduce the number of charge carriers and/orexcitons that leave the emissive layer by presenting an energy barrierthat the charge carrier or exciton has difficulty surmounting. Forexample hole transport is generally thought to be related to the HighestOccupied Molecular Orbital (HOMO) of an organic semiconductor. A “holeblocking” material may therefore be generally considered as a materialthat has a HOMO energy level significantly less than that of thematerial from which the holes are being blocked. A first HOMO energylevel is considered “less than” a second HOMO energy level if it islower on a conventional energy level diagram, which means that the firstHOMO energy level would have a value that is more negative than thesecond HOMO energy level. For example, Ir(ppy)₃ has a HOMO energy levelof −4.85 eV (data is based on the density function calculation using theB3LYP 6-31 G* basis set, Spartan 02 software package, with thepseudopetential option for materials containing heavy metals such asIr(ppy)₃). Boron 1 has a HOMO energy level of −6.49 eV, which is 1.64 eVless than that of Ir(Ppy)₃, making it an excellent hole blocker. ZrQ₄has a HOMO energy level of −5.00, only 0.15 eV less than that ofIr(ppy)₃, such that little or no hole blocking is expected. mer-GaQ₃ hasa HOMO energy level of −4.63 eV, which is greater than that of Ir(ppy)₃,such that no hole blocking at all is expected. If the emissive layerincludes different materials with different energy levels, theeffectiveness of these various materials as hole blocking layers may bedifferent, because it is the difference in HOMO energy levels betweenthe blocking layer and the blocked layer that is significant, not theabsolute HOMO energy level. But, the absolute HOMO level may be usefulin determining whether a compound will be a good hole blocker forparticular emissive layers. For example, a material having a HOMO energylevel of about −5.15 eV or less may be considered a reasonable holeblocking material for Ir(ppy)₃, which is a representative emissivematerial. Generally, a layer having a HOMO energy level that is at least0.25 eV less than that of an adjacent layer may be considered as havingsome hole blocking properties. An energy level difference of at least0.3 eV is preferred, and an energy level difference of at least 0.7 eVis more preferred. Similarly, the energy of an exciton is generallybelieved to be related to the band gap of a material. An “excitonblocking” material may generally be thought of as a material having aband gap significantly larger than the material from which excitons arebeing blocked. For example, a material having a band gap that is about0.1 eV or more, larger than that of an adjacent material, may beconsidered a good exciton blocking material.

As used herein, the term “blocking layer” means that the layer providesa barrier that significantly inhibits transport of charge carriersand/or excitons through the device, without suggesting that the layernecessarily completely blocks the charge carriers and/or excitons. Thepresence of such a blocking layer in a device may result insubstantially higher efficiencies as compared to a similar devicelacking a blocking layer. Also, a blocking layer may be used to confineemission to a desired region of an OLED.

Generally, injection layers are comprised of a material that may improvethe injection of charge carriers from one layer, such as an electrode oran organic layer, into an adjacent organic layer. Injection layers mayalso perform a charge transport function. In device 100, for example,hole injection layer 120 may be any layer comprising a material thatimproves the injection of holes from anode 115 into hole transport layer125. Exemplary materials include CuPc. In device 100, electron injectionlayer 150 may be any layer comprising a material that facilitates orenhances the injection of electrons into electron transport layer 145.Exemplary materials include LiF/Al. Other materials or combinations ofmaterials may be used for injection layers. Depending upon theconfiguration of a particular device, injection layers may be disposedat locations different than those shown in device 100. More examples ofinjection layers are provided in U.S. patent application Ser. No.09/931,948 to Lu et al., which is incorporated by reference in itsentirety. A hole injection layer may comprise a solution depositedmaterial, such as a spin-coated polymer, e.g., PEDOT:PSS, or it may be avapor deposited small molecule material, e.g., CuPc or MTDATA.

A hole injection layer (HIL) may planarize or wet the anode surface soas to provide efficient hole injection from the anode into the holeinjecting material. A hole injection layer may also have a chargecarrying component having HOMO (Highest Occupied Molecular Orbital)energy levels that favorably match up, as defined by theirherein-described relative ionization potential (IP) energies, with theadjacent anode layer on one side of the HIL and the hole transportinglayer on the opposite side of the HIL. The “charge carrying component”is the material responsible for the HOMO that actually transports holes.This component may be the base material of the HIL, or it may be adopant. Using a doped HIL allows the dopant to be selected for itselectrical properties, and the host to be selected for morphologicalproperties such as wetting, flexibility, toughness, etc. Preferredproperties for the HIL material are such that holes can be efficientlyinjected from the anode into the HIL material. In particular, the chargecarrying component of the HIL preferably has an IP not more than about0.7 eV greater that the IP of the anode material. More preferably, thecharge carrying component has an IP not more than about 0.5 eV greaterthan the anode material. Similar considerations apply to any layer intowhich holes are being injected. HIL materials are distinguished fromconventional hole transporting materials that are typically used in thehole transporting layer of an OLED in that such HIL materials may have ahole conductivity that is substantially less than the hole conductivityof conventional hole transporting materials. The thickness of the HIL ofthe present invention may be thick enough to help planarize or wet thesurface of the anode layer. For example, an HIL thickness of as littleas 10 nm may be acceptable for a very smooth anode surface. However,since anode surfaces tend to be very rough, a thickness for the HIL ofup to 50 nm may be desired in some cases.

A protective layer may be used to protect underlying layers duringsubsequent fabrication processes. For example, the processes used tofabricate metal or metal oxide top electrodes may damage organic layers,and a protective layer may be used to reduce or eliminate such damage.In device 100, protective layer 155 may reduce damage to underlyingorganic layers during the fabrication of cathode 160. Preferably, aprotective layer has a high carrier mobility for the type of carrierthat it transports (electrons in device 100), such that it does notsignificantly increase the operating voltage of device 100. CuPc, BCP,and various metal phthalocyanines are examples of materials that may beused in protective layers. Other materials or combinations of materialsmay be used. The thickness of protective layer 155 is preferably thickenough that there is little or no damage to underlying layers due tofabrication processes that occur after organic protective layer 160 isdeposited, yet not so thick as to significantly increase the operatingvoltage of device 100. Protective layer 155 may be doped to increase itsconductivity. For example, a CuPc or BCP protective layer 160 may bedoped with Li. A more detailed description of protective layers may befound in U.S. patent application Ser. No. 09/931,948 to Lu et al., whichis incorporated by reference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,a cathode 215, an emissive layer 220, a hole transport layer 225, and ananode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190, Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. patent application No. 10/233,470, which is incorporated byreference in its entirety. Other suitable deposition methods includespin coating and other solution based processes. Solution basedprocesses are preferably carried out in nitrogen or an inert atmosphere.For the other layers, preferred methods include thermal evaporation.Preferred patterning methods include deposition through a mask, coldwelding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819,which are incorporated by reference in their entireties, and patterningassociated with some of the deposition methods such as ink-jet and OVJD.Other methods may also be used. The materials to be deposited may bemodified to make them compatible with a particular deposition method.For example, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution.

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, televisions, billboards, lights forinterior or exterior illumination and/or signaling, heads up displays,fully transparent displays, flexible displays, laser printers,telephones, cell phones, personal digital assistants (PDAs), laptopcomputers, digital cameras, camcorders, viewfinders, micro-displays,vehicles, a large area wall, theater or stadium screen, or a sign.Various control mechanisms may be used to control devices fabricated inaccordance with the present invention, including passive matrix andactive matrix. Many of the devices are intended for use in a temperaturerange comfortable to humans, such as 18° C. to 30° C., and morepreferably at room temperature (20° -25° C.).

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

In an embodiment of the invention, an organic enhancement layer isprovided that is disposed between the cathode and the first organicemissive layer of an organic light emitting device. The organicenhancement layer is in direct contact with the emissive layer. In oneembodiment, the organic enhancement layer may also be in direct contactwith the cathode.

FIG. 3 shows a device 300 having an organic enhancement layer that isnot in direct contact with the cathode, because there is a separateelectron transport layer. Device 300 includes a substrate 310, an anode315, a hole injection layer 320, a hole transport layer 325, an organicemissive layer 335, an organic enhancement layer 340, an electrontransport layer 345, and a cathode 360. Cathode 360 includes a firstconductive layer 362 and a second conductive layer 364. Similarly namedlayers operate analogously to substrate 110, anode 115, hole injectionlayer 120, hole transport layer 125, emissive layer 135, electrontransport layer 145, and cathode 160 of FIG. 1.

The organic enhancement layer 340 may improve device performance. Theorganic enhancement layer 340 is not necessarily a hole blocking layer,and may have a HOMO energy level that is greater than that of emissivelayer 335, not more than 0.3 eV less than that of emissive layer 335, ornot more than 0.7 eV less than that of emissive layer 335. Where theorganic emissive layer includes multiple materials, such as a host and adopant, the HOMO energy level of the emissive layer is considered to bethat of the component that transports holes, which is generally thecomponent having the highest HOMO energy level, provided that thecomponent is present in an appreciable amount, for example about 3% orhigher. For example, in a device having an emissive layer comprising CBPdoped with Ir(ppy)₃, the HOMO level of Ir(ppy)₃, which is −4.85 eV, isthe HOMO level relevant to charge transport, because it is higher thanthe HOMO level of CBP, −5.32 eV. Without being bound to any particulartheory of how the invention works, it is believed that the organicenhancement layer 340 improves device performance by improving electroninjection into emissive layer 335. Factors that may assist in achievingthese properties include the use of a material in organic enhancementlayer 340 having a low molecular dipole moment. In some embodiments,organic enhancement layer 340 may act as a hole blocking layer, whichmay further enhance device performance, but this is not necessary.

In one embodiment of the invention, the organic enhancement layer mayinclude an aromatic hydrocarbon material comprising an aromatichydrocarbon core optionally substituted with any suitable substituent.The aromatic hydrocarbon core is an aromatic system containing noheterocyclic atoms in its rings. Suitable substituents are attached tothe aromatic hydrocarbon core. Such suitable substituents may includeelements other than hydrogen and carbon. Preferred substituents may beselected independently from the group consisting of alkyl, alkenyl,alkynyl, aryl, heteroalkyl, substituted aryl groups. Other substituentsinclude substituted heteroaryl and heterocyclic groups. In addition, twoor more of the same or different aromatic cores may be linked togetherto form a new core, which may be optionally substituted by alkyl,alkenyl, alkynyl, aryl, heteroalkyl, substituted aryl, substitutedheteroaryl, and substituted heterocyclic groups.

In a preferred embodiment of the invention, the organic enhancementlayer may include an aromatic non-heterocyclic hydrocarbon having thefollowing structure:

In another preferred embodiment of the invention, the organicenhancement layer may include an aromatic non-heterocyclic hydrocarbonhaving the following structure:

In another preferred embodiment of the invention, the organicenhancement layer may include an aromatic non-heterocyclic hydrocarbonhaving the following structure:

In another preferred embodiment of the invention, the organicenhancement layer may include an aromatic non-heterocyclic hydrocarbonhaving the following structure:

In other embodiments of the invention, the organic enhancement layer mayan aromatic non-heterocyclic hydrocarbon having the followingstructures:

In a preferred embodiment of the invention, the organic enhancementlayer may include an aromatic non-heterocyclic hydrocarbon having thefollowing structure:

such that R₁—R₅ each represent no substitution, mono-, di-, or tri-substitution, and wherein the substituents are the same or different,and each is selected from the group consisting of alkyl, alkenyl,alkynyl, aryl, heteroalkyl, and substituted aryl.

In another preferred embodiment of the invention, the organicenhancement layer may include an aromatic non-heterocyclic hydrocarbonhaving the following structure:

such that R₆—R₁₀ each represent no substitution, mono-, di-, or tri-substitution, and wherein the substituents are the same or different,and each is selected from the group consisting of alkyl, alkenyl,alkynyl, aryl, heteroalkyl, and substituted aryl

In another preferred embodiment of the invention, the organicenhancement layer may include an aromatic non-heterocyclic hydrocarbonhaving the following structure:

such that R₁₁—R₁₃ each represents no substitution, mono-, di-, or tri-substitution, and wherein R₁₁—R₁₃ are the same or differentsubstituents, and each is selected from the group consisting of alkyl,alkenyl, alkynyl, aryl, heteroalkyl, and substituted aryl;

In another preferred embodiment of the invention, the organicenhancement layer may include an aromatic non-heterocyclic hydrocarbonhaving the following structure:

such that R₁₄, R₁₉, and R₂₀—R₂₅ each represents no substitution, mono-,di-, or tri- or tetra- substitution, and R₁₅—R₁₈ each represent nosubstitution, mono-, di-, tri- substitution, and R₁₄—R₂₅ are the same ordifferent substituents, and each is selected from the group consistingof alkyl, alkenyl, alkynyl, aryl, heteroalkyl and substituted aryl.

In other embodiments of the invention, the organic enhancement layer mayinclude an aromatic non-heterocyclic hydrocarbon having the followingstructures:

such that R₂₆—R₂₈ each represents no substitution, mono-, di-, or tri-or tetra- substitution, and R₂₉—R₄₅ each represent no substitution,mono-, di-, tri- substitution, and R₂₆—R₄₅ are the same or differentsubstituents, and each is selected from the group consisting of alkyl,alkenyl, alkynyl, aryl, heteroalkyl and substituted aryl.

In other embodiments, the organic enhancement layer may include anaromatic hydrocarbon material having the following structures:

wherein R₁—R₁₀ each represent no substitution, mono-, di-, or tri-substitution, R₁₁—R₁₃, R₁₅—R₁₈, each represents no substitution, mono-,di-, or tri- substitution, and R₁₄, R₁₉, and R₂₀—R₂₅ each represents nosubstitution, mono-, di-, or tri- or tetra- substitution, and

-   -   and wherein R₁₁—R₂₅ are the same or different substituents, and        each is selected from the group consisting of alkyl, alkenyl,        aryl, heteroalkyl, substituted aryl, substituted heteroaryl and        heterocyclic groups.

In another embodiment of the invention, the organic enhancement layermay include an aromatic hydrocarbon material having a core comprising atleast 3 phenyl rings, wherein each phenyl ring in the core is fused orattached by a single C—C bond to at least one other phenyl ring in thecore, and wherein the core is contiguous. In a contiguous core, eachphenyl ring in the core is connected to every other phenyl ring in thecore either directly, or by a chain of phenyl rings that are eitherfused or attached by a single C—C bond. Examples of ways that phenylrings may be attached to other phenyl rings in a contiguous core areprovided below:

In another embodiment, the organic enhancement layer may include anaromatic hydrocarbon material having a core comprising at least 6 phenylrings, wherein each phenyl ring in the core is fused or attached by asingle C—C bond to at least one other phenyl ring in the core, andwherein the core is contiguous.

In another embodiment, a “core” may be defined as a group of least threephenyl rings, where each phenyl ring is π-conjugated to every otherphenyl ring in the group. In this embodiment, a broader range ofconnections may be used between the phenyl rings, such as a chain ofunsaturated carbon bonds.

In a typical phosphorescent device, it is believed that excitons areformed when electrons and holes meet and recombine in the emissivelayer. It is also believed that this recombination generally occurs veryclose to where the electrons are injected into the emissive region.There are several possible reasons for this phenomenon. First,phosphorescent emissive materials may have superior hole transportproperties, such that a phosphorescent emissive layer has a higher holemobility than electron mobility, and holes are quickly transportedacross the emissive layer, as compared to electrons, which are moreslowly transported. Second, holes may be injected into the emissivelayer more readily than electrons. “Hole blocking” layers may be used toprevent holes from leaving the emissive layer. Some degree of holeblocking may occur where the hole blocking layer has a HOMO energy levellower than that of the adjacent layer, usually an emissive layer, fromwhich holes are being blocked. Effective hole blocking layers generallyhave a HOMO energy level significantly lower than that of the adjacentlayer, such as 0.25 eV lower, preferably 0.3 eV lower or more preferably0.7 eV lower. The following table lists density function calculations(DFT) performed using the Spartan 02 software package withB31LYP/CEP-31G* basis set, with the pseudopotential option for materialscontaining heavy metals such as Ir(ppy)₃ or the Gaussian 98 softwarepackage with the B31LYP/CEP-31G* basis set where indicated by the symbol“#”. TABLE I DFT Compound HOMO (eV) LUMO (eV) Dipole (D) I −5.66 −1.010.24 II −5.66 −1.05 0.13 HPT −5.54 −1.29 0.01 TSBF −5.21 −1.37 0.28 CBP−5.32 −1.23 0.00 fac-Ir(ppy)₃ −4.85 −1.21 6.14 fac-Ir(ppy)₃# −4.93 −1.436.53 fac-Ir(5-Phppy)₃# −4.96 −1.56 7.49 TPBi −5.70 −1.25 5.63 BCP −5.87−1.17 2.89

Without necessarily being limited as to any particular theory as to howthe invention works, it is believed that the organic enhancement layermay allow for efficient injection of electrons into the emissive layer.Instead of or in addition to “blocking” holes which then wait forelectrons to trickle into the emissive layer, the organic enhancementlayer may result in a flood of electrons that recombine with holesbefore they can reach the edge of the emissive layer. Superior electroninjection properties may result from one of several properties of theorganic enhancement layer material or materials.

In a preferred embodiment, the organic enhancement layer acts as both ahole blocking layer and provides superior electron injection. Forexample, compounds I and II have a HOMO that is 0.81 eV less than theHOMO of Ir(ppy)₃, a desirable phosphorescent emissive material. In adevice where Ir(ppy)₃ is both the emissive material and provides thehole transport function in the emissive layer, for example, an organicenhancement layer of compound I or II may block holes due to the largeHOMO level difference of 0.81 eV, and may also provide superior electroninjection. Depending upon the hole transport properties of the emissivelayer, other HOMO energy level differences may be utilized in accordancewith embodiments of the invention to achieve both hole blocking andsuperior electron injection.

In one embodiment, the use of an organic enhancement layer including anaromatic hydrocarbon material having a low molecular dipole moment mayresult in superior device performance. Without intending to limit allembodiments with a particular theory of how the invention works, it isbelieved that a low molecular dipole moment may improve electroninjection from the organic layer into the emissive layer, because thepresence of a significant molecular dipole moment may lead to a localelectric field that can trap or slow down charge migration. As a result,the material may inject electrons into the emissive layer rapidly enoughthat a significant proportion of holes in the organic emissive layerencounter an electron and recombine before reaching the interfacebetween the organic enhancement layer and the emissive layer. In someembodiments of the invention, the second organic layer does not haveenergetics suitable for hole blocking, because hole blocking may not beneeded due to superior electron injection. The organic enhancement layermay have a HOMO energy level that is not generally considered effectiveas a hole blocking layer. For example, the HOMO energy level of thesecond organic layer may be greater than that of the emissive layer, inwhich case no hole blocking at all would be expected. By way of furtherexample, the HOMO energy level of the organic enhancement layer may beless than that of the emissive layer, but the difference may be so smallthat significant hole blocking is not expected. For example, thedifference in energy levels may be less than about 0.25 eV, about 0.3eV, or about 0.7 eV.

In one embodiment, it is believed that the use of an organic enhancementlayer including an aromatic hydrocarbon having a zero or low moleculardipole moment may result in superior device performance. Withoutintending to limit all embodiments with a particular theory of how theinvention works, it is believed that this symmetric energy structure mayimprove electron injection from the organic enhancement layer into theemissive layer because the symmetry in the HOMO and LUMO of the aromatichydrocarbon molecules may lead to reduce dipole induced charge trapsites, thus better charge hopping between the molecules as compared toheterocyclic analogs which may have localized electron densityassociated with heterocyclic systems. In addition, it is also believedthat the lack of heteroatoms and delocalized electron density in thecore structures of the aromatic hydrocarbons of the present inventionlead to a better delocalization and stabilization when the aromatichydrocarbon is oxidized (cation radical) or reduced (anion radical).This results in a lower susceptibility to bond breaking in the aromatichydrocarbons, and thus higher stability in devices incorporating thesematerials compared to heterocyclic aromatics. Further, the lack of polarsubstituents such as halogen (I, Br, Cl, and F), and CN on the aromatichydrocarbon may serve the same purpose since polar groups tend toperturb the electron density distribution on the aromatic ring, makingthe bonds on the ring more susceptible to rupture. Superior electroninjection may enable superior device performance without necessarilyusing a blocking layer, as described above with respect to low moleculardipole moment materials.

“Stability” may be measured in a number of ways. One stabilitymeasurement is the operational stability of the electroluminescentdevice. The operational half-life is the time required for the luminanceof the device to decay from the initial luminance (Lo) to 50% of itsinitial luminance (L_(0.5)) under constant direct current drive and atroom temperature unless otherwise noted. Operational half-life dependsupon luminance at which the device is operated, because a higherluminance generally corresponds to a faster decay in a particulardevice. Devices in accordance with embodiments of the present inventioncan advantageously have an operational half-life in excess of about 1000hours at an initial luminance of about 100 to about 1000 cd/m².

In another embodiment, the organic enhancement layer may comprise amaterial having a low molecular dipole moment, wherein a low moleculardipole moment means a molecular dipole moment of less than about 2.0debyes, such that the device has an unmodified external quantumefficiency of at least about 3%; a highest unoccupied molecular orbitalthat is not more than 0.8 eV less than the highest unoccupied molecularorbital of the hole transporting material in the adjacent organic layer;and a lifetime of at least about 1000 hours at an initial luminance ofabout 100 to about 1000 cd/m². In another embodiment, the organicenhancement layer may comprise a material having a highest unoccupiedmolecular orbital that is at least about 0.8 eV greater than the highestoccupied molecular orbital of the hole transporting material in theadjacent first organic layer, such that the device has an unmodifiedexternal quantum efficiency of at least about 5% and a lifetime of atleast about 1000 hours at an initial luminance of about 100 to about1000 cd/m². In yet another embodiment, the organic enhancement layer maycomprise a material having a molecular dipole moment less than about 2.0debyes, such that the device has an external quantum efficiency of atleast about 5% at from about 100 to about 1000 cd/m². The organicenhancement layer may be in direct contact with the cathode, or theremay be a separate organic layer between the organic enhancement layerand the cathode. Other aromatic hydrocarbon materials may be used.

As used herein, the term “external quantum efficiency” refers to thepercentage of charge carriers injected into a device that result in theemission of a photon from the device in the forward direction. A numberof factors can affect the external quantum efficiency, including the“internal quantum efficiency,” which is the percentage of chargecarriers injected into a device that result in the creation of a photon,and the “outcoupling efficiency,” which is the percentage of photonscreated that are emitted from a device towards a viewer. Manyembodiments of the present invention are directed to an organic layercomprising an aromatic hydrocarbon layer that is in direct contact withan emissive layer, which may enhance the internal quantum efficiency andthus the external quantum efficiency of the device. Because externalquantum efficiency is more readily and directly measured than internalquantum efficiency, it may be desirable to describe certain aspects ofthe invention with respect to external quantum efficiency. But, in orderto determine whether an enhanced external quantum efficiency is due tothe use of an aromatic hydrocarbon, it is preferable to account forother factors that affect external quantum efficiency. The term“unmodified external quantum efficiency” as used herein refers to theexternal quantum efficiency of a device, after multiplication by afactor to account for any differences in the outcoupling efficiency ofthat device and the outcoupling efficiency of the devices describedexperimentally herein. For example, a device having an external quantumefficiency of 5%, but having an outcoupling efficiency 3 times betterthan the devices described herein, would have an “unmodified externalquantum efficiency” of 1.33% (one third of 5%). A typical outcouplingefficiency for the types of devices described herein is about 20-30%.There are device structures having better outcoupling efficiencies thanthe devices described herein, and it is anticipated that improvements tooutcoupling efficiency will be made over time. Such improvements wouldenhance external quantum efficiency, but should not affect “unmodified”external quantum efficiency, and devices having such improvements mayfall within the scope of the present invention.

As electrons are expected to reside primarily on the LUMO, moleculeswith higher symmetry may show greater delocalization of the negativecharge over the molecule. The enhanced delocalization of charge mayincrease the bulk conductivity, electron mobility and operationalstability properties in a device.

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. It is understood thatvarious theories as to why the invention works are not intended to belimiting. For example, theories relating to charge transfer are notintended to be limiting. While many embodiments of the invention allowfor superior device performance in the absence of hole blocking, it isunderstood that some embodiments of the invention may be combined withhole blocking.

Material Definitions:

As used herein, abbreviations refer to materials as follows:

-   CBP: 4,4′-N,N-dicarbazole-biphenyl-   m-MTDATA 4,4′,4′-tris(3-methylphenylphenlyamino)triphenylamine-   Alq₃: aluminum(III) tris(8-hydroxyquinolate)-   Bphen: 4,7-diphenyl-1,10-phenanthroline-   n-BPhen: n-doped BPhen (doped with lithium)-   F₄-TCNQ: tetrafluoro-tetracyano-quinodimethane-   p-MTDATA: p-doped m-MTDATA (doped with F₄-TCNQ)-   Ir(pPY)₃: tris(2-phenylpyridine)-iridium-   Ir(ppz)₃: tris(1-phenylpyrazoloto,N,C(2′)iridium(III)-   BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline-   TAZ: 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole-   CuPc: copper phthalocyanine.-   ITO: indium tin oxide-   NPD: N,N′-diphenyl-N-N′-di(1-naphthyl)-benzidine-   TPD: N,N′-diphenyl-N,N′-di(3-toly)-benzidine-   BAlq:    aluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate-   mCP: 1,3-N,N-dicarbazole-benzene-   DCM: 4-(dicyanoethylene)-6-(4-dimethylaminostyryl-2-methyl)-4H-pyran-   DMQA: N,N′-dimethylquinacridone-   PEDOT:PSS: an aqueous dispersion of poly(3,4-ethylenedioxythiophene)    with polystyrenesulfonate (PSS)-   ZrQ₄: zirconium(IV) tetrakis(8-hydroxyquinolate)-   HfQ₄: hafnium(IV) tetrakis(8-hydroxyquinolate)-   GaQ₃: gallium(III) tris(8-hydroxyquinolate)-   PBD: 2-(4-biphenylyl)-5-phenyl-1,3,4-oxidiazole-   TPBi: 2,2′,2″-(1,3,5-benzenetriyl)tris-(1-phenyl-1H-benzimidazole)-   Boron 1: 1,4-Bis(diphenylboronyl)benzene-   Boron 2: Tris(2,3,5,6-tetramethylphenyl)borane-   HPT: 2,3,6,7,10,11-hexaphenyltriphenylene-   TSBF: ter-2,7-spirobifluorene-   Ir(5-Phppy)₃: tris[5-phenyl(2-phenylpyridine)]iridium(III)-   Ir(3′-Mepq)₂(acac) bis[3′-methyl(2-phenylquinoline)]iridium(III)    acetylacetonate

EXPERIMENTAL

Specific representative embodiments of the invention will now bedescribed, including how such embodiments may be made. It is understoodthat the specific methods, materials, conditions, process parameters,apparatus and the like do not necessarily limit the scope of theinvention.

EXAMPLE 1 Synthesis of 3′,5′-diphenyl-p-quaterphenyl (Compound II)

Step 1

Preparation of Sodium 4-Biphenylacetate

4-Biphenylacetic acid (25.0 g, 0.1177 mol.) and sodium hydroxide (4.71g, 0.1177 mol.) were dissolved in 50 mL of water and stirred at roomtemperature for 2 hours. The product was isolated by evaporating off thewater.

Step 2

Preparation of 3′,5′-diphenyl-p-quaterphenyl

Sodium 4-biphenylacetate (25.0 g, 0.106 mol.) was placed in a suspensionof 2,4,6-triphenylpyrylium tetrafluoroborate (41.0 g, 0.106 mol.) in 80mL of acetic anhydride under nitrogen atmosphere and refluxed for 24hours. The crude product was extracted with a 7:3 ratio mixture ofhexane/dichloromethane. The solvent was stripped, and the crude productsublimed 4 times to achieve 99% purity. The practical yield followingsublimations was 20%.

EXAMPLE 2-3 and COMPARATIVE EXAMPLES 4-5

Experimental Device Fabrication

All devices were fabricated using thermal evaporation at a pressure of<1×10⁻⁷ Torr. The devices were fabricated on a glass substrate precoatedwith a 120 nm thick anode of indium tin oxide (ITO), commerciallyavailable from Applied Films of Longmont, Colo. The cathode was 1 nm ofLiF followed by 100 nm of aluminum. All devices were encapsulated with aglass lid sealed with an epoxy resin under nitrogen (<1 ppm H₂O and O₂)immediately after fabrication, and a moisture getter was incorporatedinside each device. Device lifetime is defined as the time required forthe luminance to decrease from its initial value to 50% of the initialvalue, at room temperature under constant DC drive.

Current-voltage-luminance characteristics and operational lifetime weremeasured and are summarized in Table II, below. A typical displaybrightness level of 600 cd/m² for green emitting devices was chosen forpurposes of comparison between devices.

The organic stack was fabricated to consist of CuPc as a hole injectionlayer at a thickness of 10 nm; NPD as a hole transport layer at athickness of 30 nm; CBP doped with 6 wt % Ir(ppy)₃ as the emissive layerat a thickness of 30 nm. Adjacent to the emissive layer was an electrontransport layer, ETL2, consisting of 10 nm of an aromatic hydrocarbon(Compounds I and II described above) or, for the comparative examples, aheterocyclic aromatic hydrocarbon. Adjacent to layer ETL2 was anelectron transport layer consisting of Alq₃ at a thickness of 40 nm.TABLE II Efficiency % luminance retained (cd/A, %) at at 100 hours atinitial Example ETL2 material 600 cd/m² luminance of 600 cd/m² 2Compound I 21, 6.0 95 3 Compound II 29, 8.2 90 4 TPBi 22, 6.1 75 5 BCP35, 9.7 85Additional testing of the device of example 3 yielded a retention of 70%of the initial luminance at 650 hours (initial luminance was 600 cd/m²).

EXAMPLE 6 Synthesis of 2,3,6,7,10,11-hexaphenyltriphenylene (HPT)

Step 1

Preparation of 2,3,6,7,10,11-hexabromotriphenylene

2,3,6,7,10,11-Hexabromotriphenylene was prepared according to theliterature method (Breslow et al, Tetrahedron, 1982, 38, 863).Triphenylene (3.0 g, 13.2 mmol) was dissolved in 70 mL of nitrobenzene.0.27 g of Fe powder was added. To this mixture, bromine (18.6 g, 120mmol) in 20 mL of nitrobenzene was added via a dropping funnel. Themixture was stirred at room temperature for 12 hours and brought toreflux for 2 hours. After cooling, the solid was filtered, washed withethanol and dried. 8.9 g (96%) of crude product was obtained.Recrystallization in boiling 1,2-dichlorobenzene (˜180° C.) yielded theproduct as off-white needles (8.64 g, 94%). The product was confirmed bymass spectrometry.

Step 2

Preparation of 2,3,6,7,10,11-hexaphenyltriphenylene (HPT)

2,3,6,7,10,11-hexabromotriphenylene (8.64 g, 12.3 mmol), phenylboronicacid (13.5 g, 111 mmol), triphenylphosphine (0.64 g, 2.46 mmol),Pd(OAc)₂ (0.14 g, 0.615 mmol), K₂CO₃ (20.4 g, 147.6 mmol) were in 600 mLof xylenes and 50 mL of water. The mixture was purged with nitrogen for5 minutes and slowly brought to reflux under nitrogen. TLC(CH₂Cl₂:hexane ˜1:2 v/v, the starting hexabromo compound did not move)showed the appearance of a new spot (xylenes also showed up on the TLCbut it eluted up faster than the product) within hours. The mixture wasrefluxed for 12 hours. After cooling, the solid was filtered, washedwith ethanol and dried. The crude yield was higher than 90%.Recrystallization in xylenes (˜100 mL per 3-4 g of product) yield whitecrystals. Vacuum sublimation yielded the product (˜4.5 g) which wasconfirmed by NMR and mass spectrometry.

EXAMPLE 7 Synthesis of ter-2,7-spirobifluoroene (TSBF)

TSBF was synthesized according to Wong, et. al. J. Am. Chem. Soc., 2002,124, 11576-11577. The product was purified by vacuum sublimation.

Device Fabrication and Measurement

All devices were fabricated by high vacuum (<10⁻⁷ Torr) thermalevaporation. The anode electrode was ˜1200 Å of indium tin oxide (ITO).The cathode consisted of 10 Å of LiF followed by 1,000 Å of Al. Alldevices were encapsulated with a glass lid sealed with an epoxy resin ina nitrogen glove box (<1 ppm of H₂O and O₂) immediately afterfabrication, and a moisture getter was incorporated inside the package.

The current-voltage-luminance (IVL) characteristics were measured. Theluminous efficiency and operational lifetime are summarized in the TableIII. A typical display brightness level of 600 cd/m² is chosen for thecomparison between different devices. Device operational stability wasmeasured at room temperature under constant direct current of 40 mA/cm²or at initial luminance (L₀) of 955 nits; or at 60° C. under constantdirect current at L₀=955 cd/m². The halflife (T_(1/2)) is defined as thetime required for the Lo to drop to 50% of L₀.

EXAMPLE 8

The organic stack consisted of 100 Å thick of copper phthalocyanine(CuPc) as the hole injection layer (HIL), 300 Å of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as the holetransporting layer (HTL), 300 Å of 4,4′-bis(N-carbazolyl)biphenyl (CBP)doped with 4.5 wt % of Ir(5-Phppy)₃ as the emissive layer (EML). TheETL2 was 100 Å of HPT. The ETL1 was 400 Å oftris(8-hydroxyquinolinato)aluminum (Alq₃).

EXAMPLE 9

The organic stack consisted of 100 Å thick of copper phthalocyanine(CuPc) as the hole injection layer (HIL), 300 Å of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(α-NPD), as the holetransporting layer (HTL), 300 Å of 4,4′-bis(N-carbazolyl)biphenyl (CBP)doped with 6 wt % of Ir(5-Phppy)₃ as the emissive layer (EML). The ETL2was 100 Å of HPT. The ETL1 was 400 Å oftris(8-hydroxyquinolinato)aluminum (Alq₃).

EXAMPLE 10

The organic stack consisted of 100 Å thick of copper phthalocyanine(CuPc) as the hole injection layer (HIL), 300 Å of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as the holetransporting layer (HTL), 300 Å of 4,4′-bis(N-carbazolyl)biphenyl (CBP)doped with 12 wt % of Ir(5-Phppy)₃ as the emissive layer (EML). The ETL2was 100 Å of HPT. The ETL1 was 400 Å oftris(8-hydroxyquinolinato)aluminum (Alq₃).

COMPARATIVE EXAMPLE 11

The organic stack consisted of 100 Å thick of copper phthalocyanine(CuPc) as the hole injection layer (HIL), 300 Å of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as the holetransporting layer (HTL), 300 Å of 4,4′-bis(N-carbazolyl)biphenyl (CBP)doped with 4.5 wt % of Ir(5-Phppy)₃ as the emissive layer (EML). TheETL2 was 100 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq).The ETL1 was 400 Å of tris(8-hydroxyquinolinato)aluminum (Alq₃).

EXAMPLE 12

The organic stack consisted of 100 Å thick of copper phthalocyanine(CuPc) as the hole injection layer (HIL), 300 Å of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as the holetransporting layer (HTL), 300 Å of 4,4′-bis(N-carbazolyl)biphenyl (CBP)doped with 12 wt % of Ir(5-Phppy)₃ as the emissive layer (EML). The ETL2was 400 Å of HPT. There was no ETL1.

EXAMPLE 13

The organic stack consisted of 100 Å thick of copper phthalocyanine(CuPc) as the hole injection layer (HIL), 400 Å of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as the holetransporting layer (HTL), 300 Å of 4,4′-bis(N-carbazolyl)biphenyl (CBP)doped with 12 wt % of Ir(3′-Mepq)₂(acac) as the emissive layer (EML).The ETL2 was 100 Å of HPT. The ETL1 was 400 Å oftris(8-hydroxyquinolinato)aluminum (Alq₃).

COMPARATIVE EXAMPLE 14

The organic stack consisted of 100 Å thick of copper phthalocyanine(CuPc) as the hole injection layer (HIL), 400 Å of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as the holetransporting layer (HTL), 300 Å of 4,4′-bis(N-carbazolyl)biphenyl (CBP)doped with 12 wt % of Ir(3′-Mepq)₂(acac) as the emissive layer (EML).The ETL2 was 150 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq).The ETL1 was 400 Å of tris(8-hydroxyquinolinato)aluminum (Alq₃).

EXAMPLE 15

The organic stack consisted of 100 Å thick of copper phthalocyanine(CuPc) as the hole injection layer (HIL), 300 Å of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as the holetransporting layer (HTL), 300 Å of 4,4′-bis(N-carbazolyl)biphenyl (CBP)doped with 4.5 wt % of Ir(5-Phppy)₃ as the emissive layer (EML). TheETL2 was 400 Å of TSBF. There was no ETL1.

EXAMPLE 16

The organic stack consisted of 100 Å thick of copper phthalocyanine(CuPc) as the hole injection layer (HIL), 300 Å of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as the holetransporting layer (HTL), 300 Å of 4,4′-bis(N-carbazolyl)biphenyl (CBP)doped with 4.5 wt % of Ir(5-Phppy)₃ as the emissive layer (EML). TheETL2 was 100 Å of TSBF. The ETL1 was 300 Å ofaluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate (BAlq).

EXAMPLE 7

The organic stack consisted of 100 Å thick of copper phthalocyanine(CuPc) as the hole injection layer (HIL), 300 Å of4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (α-NPD), as the holetransporting layer (HTL), 300 Å of 4,4′-bis(N-carbazolyl)biphenyl (CBP)doped with 4.5 wt % of Ir(5-Phppy)₃ as the emissive layer (EML). TheETL2 was 100 Å of TSBF. The ETL1 was 300 Å oftris(8-hydroxyquinolinato)aluminum (Alq₃). TABLE III Luminous ETL2 ETL1Efficiency (cd/A) T_(1/2) (hour) at Example Material Material at 600cd/m² J = 40 mA/cm²  8 HPT Alq 37.4 350  9 HPT Alq 40.3 377 10 HPT Alq43.4 234 Comparative 11 BAlq Alq 28.1 350 12 HPT none 18.3 not tested 13HPT Alq 14.3 100 Comparative 14 BAlq Alq 13.6 250 15 TSBF none 18.2  1416 TSBF BAlq 16.7 300 17 TSBF Alq 26.7 140

FIG. 5 shows luminous efficiency vs. brightness for examples 8-10 andcomparative example 11. FIG. 6 shows external quantum efficiency vs.current density for examples 8-10 and comparative example 11. Higherefficiencies are demonstrated by examples 8-10 which utilize HPT as theETL2 compared to example 11 which utilizes BAlq as the ETL2.

FIG. 7 shows the normalized electroluminescence spectra for examples8-10 which utilize HPT as the ETL2. There is no change in the spectraupon varying the dopant concentration of Ir(5-Phppy)₃ in the EML from4.5% to 12%.

FIG. 8 shows the operational stability for examples 8-10 which utilizeHPT as the ETL2 at room temperature under constant direct current of 40mA/cm² and at initial luminance (L₀) of 955 nits.

FIG. 9 shows the operational stability for examples 8-10 which utilizeHPT as the ETL2 at 60° C. under constant direct current at L₀=955 cd/m².

FIG. 10 shows the current-voltage characteristics for example 12. Highervoltage is required to operate compared to examples 8-10.

FIG. 11 shows the luminous efficiency vs. brightness for example 12.FIG. 12 shows the external quantum efficiency vs. current density forexample 12. Lower efficiencies are obtained compared to examples 8-10.

FIG. 13 shows the current-voltage characteristics for examples 13 andcomparative example 14. Lower voltage is required to operate example 13which utilize HPT as the ETL2 compared to example 14 which utilizes BAlqas the ETL2.

FIG. 14 shows luminous efficiency vs. brightness for example 13 andcomparative example 14. FIG. 15 shows external quantum efficiency vs.current density for example 13 and comparative example 14. Higherefficiencies are demonstrated by example 13 which utilize HPT as theETL2 compared to example 14 which utilizes BAlq as the ETL2.

FIG. 16 shows the normalized electroluminescence spectra of example 13which utilize HPT as the ETL2.

FIG. 17 shows the operational stability of examples 13 which utilize HPTas the ETL2 at room temperature under constant direct current of 40mA/cm².

FIG. 18 shows the current-voltage characteristics of examples 15-17.Lowest voltage is required to operate example 17 which utilizes TSBF asthe ETL2 and Alq as the ETL1. Highest voltage is required to operateexample 15 which utilize TSBF as the ETL2 and there is no ETL1.

FIG. 19 shows luminous efficiency vs. brightness for examples 15-17.FIG. 20 shows external quantum efficiency vs. current density forexamples 15-17. Similar efficiency is obtained in example 17 whichutilizes TSBF as the ETL2 compared to example 11 which utilizes BAlq asthe ETL2.

FIG. 21 shows the normalized electroluminescence spectra of examples15-17 which utilize TSBF as the ETL2.

FIG. 22 shows the operational stability of examples 15-17 which utilizesTSBF as the ETL2 at room temperature under constant direct current of 40mA/cm².

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. The presentinvention as claimed therefore includes variations from the particularexamples and preferred embodiments described herein, as will be apparentto one of skill in the art.

1. A device, comprising: an anode; a cathode; a first organic layerdisposed between the anode and the cathode, wherein the first organiclayer comprises a material that produces phosphorescent emission when avoltage is applied between the anode and the cathode; and a secondorganic layer disposed between the first organic layer and the cathode,wherein the second organic layer is in direct contact with the firstorganic layer, and wherein the second organic layer comprises anaromatic non-heterocyclic hydrocarbon material.
 2. The device of claim1, wherein the aromatic hydrocarbon material has a molecular dipolemoment of less than about 2.0 debyes.
 3. The device of claim 2, whereinthe aromatic hydrocarbon material has a molecular dipole moment of zero.4. The device of claim 1, wherein the aromatic hydrocarbon materialcomprises a material having the structure:

wherein: R₁—R₅ each represent no substitution, mono-, di-, or tri-substitution, and wherein the substituents are the same or different,and each is selected from the group consisting of alkyl, alkenyl,alkynyl, aryl, heteroalkyl and substituted aryl.
 5. The device of claim4, wherein the aromatic hydrocarbon material has the structure:


6. The device of claim 1, wherein the aromatic hydrocarbon material hasthe structure:

wherein: R₆—R₁₀ each represent no substitution, mono-, di-, or tri-substitution, and wherein the substituents are the same or different,and each is selected from the group consisting of alkyl, alkenyl,alkynyl, aryl, heteroalkyl, and substituted aryl.
 7. The device of claim6, wherein the aromatic comprises a hydrocarbon material having thestructure:


8. The device of claim 1, wherein the second organic layer is in directcontact with the cathode.
 9. The device of claim 1, further comprising athird organic layer disposed between the second organic layer and thecathode.
 10. The device of claim 1, wherein the aromatic hydrocarbonmaterial has a highest occupied molecular orbital that is not more than0.81 eV less than the highest occupied molecular orbital of the holetransporting material in the first organic layer.
 11. The device ofclaim 10, wherein the aromatic hydrocarbon material has a moleculardipole moment less than about 2.0 debyes.
 12. A device, comprising: ananode; a cathode; an first organic layer disposed between the anode andthe cathode, wherein the first organic layer comprises a material thatproduces phosphorescent emission when a voltage is applied between theanode and the cathode; a second organic layer disposed between the firstorganic layer and the cathode, wherein the second organic layer is indirect contact with the first organic layer, and wherein the secondorganic layer comprises an aromatic non-heterocyclic hydrocarbonmaterial having a highest occupied molecular orbital that is at least0.81 eV less than the highest occupied molecular orbital of the holetransporting material in the first organic layer.
 13. The device ofclaim 12, wherein the aromatic hydrocarbon material has a moleculardipole moment less than about 2.0 debyes.
 14. A device, comprising: ananode; a cathode; an first organic layer disposed between the anode andthe cathode, wherein the first organic layer is comprises a materialthat produces phosphorescent emission when a voltage is applied betweenthe anode and the cathode; a second organic layer disposed between thefirst organic layer and the cathode, wherein the second organic layer isin direct contact with the first organic layer, and wherein the secondorganic layer comprises an aromatic non-heterocyclic hydrocarbonmaterial, and wherein the device has an unmodified external quantumefficiency of at least about 3% and a lifetime of at least about 1000hours at an initial luminance of about 100 to about 1000 cd/m².
 15. Thedevice of claim 14, wherein the device has an unmodified externalquantum efficiency of at least about 5% and a lifetime of at least about1000 hours at an initial luminance of about 100 to about 1000 cd/m². 16.The device of claim 14, wherein the aromatic hydrocarbon material has amolecular dipole moment less than about 2.0 debyes.
 17. The device ofclaim 14, wherein the aromatic hydrocarbon material has a zero moleculardipole moment.
 18. The device of claim 14, wherein the aromatichydrocarbon material has the structure:

wherein: R₁—R₅ each represent no substitution, mono-, di-, or tri-substitution, and wherein the substituents are the same or different,and each is selected from the group consisting of alkyl, alkenyl,alkynyl, aryl, heteroalkyl, and substituted aryl.
 19. The device ofclaim 18, wherein the aromatic hydrocarbon material has the structure:


20. The device of claim 14, wherein after 100 hours of operation at aninitial luminance of 600 cd/m² at least about 90% of initial luminanceis retained.
 21. The device of claim 14, wherein after 1000 hours ofoperation at an initial luminance of 1000 cd/m² at least about 70% ofinitial luminance is retained.
 22. The device of claim 14, wherein thearomatic hydrocarbon material has the structure:

wherein: R₆—R₁₀ each represent no substitution, mono-, di-, or tri-substitution, and wherein the substituents are the same or different,and each is selected from the group consisting of alkyl, alkenyl,alkynyl, aryl, heteroalkyl, and substituted aryl.
 23. The device ofclaim 22, wherein the aromatic hydrocarbon material has the structure:


24. The device of claim 1, wherein the aromatic hydrocarbon material hasthe structure:

wherein: R₁₁—R₁₃ each represents no substitution, mono-, di-, or tri-substitution, and wherein the substituents are the same or different,and each is selected from the group consisting of alkyl, alkenyl,alkynyl, aryl, heteroalkyl, and substituted aryl.
 25. The device ofclaim 24, wherein R₁₁, R₁₂, and R₁₃ are each phenyl.
 26. The device ofclaim 25, wherein the aromatic hydrocarbon material has the structure:


27. The device of claim 24, wherein the aromatic hydrocarbon materialhas the structure


28. The device of claim 1, wherein the aromatic hydrocarbon material hasthe structure:

wherein: R₁₄, R₁₉, and R₂₀—R₂₅ each represents no substitution, mono-,di-, or tri- or tetra- substitution, and R₁₅—R₁₈ each represent nosubstitution, mono-, di-, tri- substitution, and each R substituent isthe same or different, and each is selected from the group consisting ofalkyl, alkenyl, alkynyl, aryl, heteroalkyl and substituted aryl.
 29. Thedevice of claim 28, wherein the aromatic hydrocarbon material has thestructure


30. The device of claim 1, wherein the aromatic hydrocarbon material hasthe structure:

wherein R₂₆ represents no substitution, mono-, di-, tri-, or tetrasubstitution, and wherein each R is the same or different substituent,and each is selected from the group consisting of alkyl, alkenyl,alkynyl, aryl, heteroalkyl and substituted aryl, and each R may belinked together to form cyclic substituents such as cycloalkyl oraromatic non-heterocyclic rings.
 31. The device of claim 30, wherein thearomatic hydrocarbon material has the structure


32. The device of claim 1, wherein the aromatic hydrocarbon material hasthe structure

wherein R₂₇ and R₂₈ each represents no substitution, mono-, di-, tri-,or tetra substitution, and wherein R₂₇ and R₂₈ are the same or differentsubstituents, and each is selected from the group consisting of alkyl,alkenyl, alkynyl, aryl, heteroalkyl and substituted aryl, and each R maybe linked together to form cyclic substituents such as cycloalkyl oraromatic non-heterocyclic rings.
 33. The device of claim 32, wherein thearomatic hydrocarbon material has the structure


34. The device of claim 1, wherein the aromatic hydrocarbon material hasthe structure

wherein R₂₉—R₃₁ each represents no substitution, mono-, di-, or tri-substitution, and wherein R₂₉—R₃₁ are the same or differentsubstituents, and each is selected from the group consisting of alkyl,alkenyl, alkynyl, aryl, heteroalkyl and substituted aryl, and each R maybe linked together to form cyclic substituents such as cycloalkyl oraromatic non-heterocyclic rings.
 35. The device of claim 34, wherein thearomatic hydrocarbon material has the structure


36. The device of claim 1, wherein the aromatic hydrocarbon material hasthe structure

wherein R₃₂—R₃₄ each represents no substitution, mono-, di-, or tri-substitution, and wherein R₃₂—R₃₄ are the same or differentsubstituents, and each is selected from the group consisting of alkyl,alkenyl, alkynyl, aryl, heteroalkyl and substituted aryl, and each R maybe linked together to form cyclic substituents such as cycloalkyl oraromatic non-heterocyclic rings.
 37. The device of claim 36, wherein thearomatic hydrocarbon material has the structure


38. The device of claim 1, wherein the aromatic hydrocarbon material hasthe structure

wherein R₃₅—R₃₇ each represents no substitution, mono-, di-, or tri-substitution, and wherein R₃₅—R₃₇ are the same or differentsubstituents, and each is selected from the group consisting of alkyl,alkenyl, alkynyl, aryl, heteroalkyl and substituted aryl, and each R maybe linked together to form cyclic substituents such as cycloalkyl oraromatic non-heterocyclic rings.
 39. The device of claim 38, wherein thearomatic hydrocarbon material has the structure


40. The device of claim 1, wherein the aromatic hydrocarbon material hasthe structure

wherein R₃₈—R₄₁ each represents no substitution, mono-, di-, or tri-substitution, and wherein R₃₈—R₄₁ are the same or differentsubstituents, and each is selected from the group consisting of alkyl,alkenyl, alkynyl, aryl, heteroalkyl and substituted aryl, and each R maybe linked together to form cyclic substituents such as cycloalkyl oraromatic non-heterocyclic rings.
 41. The device of claim 40, wherein thearomatic hydrocarbon material has the structure


42. The device of claim 1, wherein the aromatic hydrocarbon material hasthe structure

wherein R₄₂—R₄₅ each represents no substitution, mono-, di-, or tri-substitution, and wherein R₄₂—R₄₅ are the same or differentsubstituents, and each is selected from the group consisting of alkyl,alkenyl, alkynyl, aryl, heteroalkyl and substituted aryl, and each R maybe linked together to form cyclic substituents such as cycloalkyl oraromatic non-heterocyclic rings.
 43. The device of claim 42, wherein thearomatic hydrocarbon material has the structure


44. The device of claim 1, wherein the second organic layer is in directcontact with the cathode.
 45. A device, comprising: an anode; a cathode;a first organic layer disposed between the anode and the cathode,wherein the first organic layer comprises a material that producesphosphorescent emission when a voltage is applied between the anode andthe cathode; and a second organic layer disposed between the firstorganic layer and the cathode, wherein the second organic layer is indirect contact with the first organic layer, and wherein the secondorganic layer comprises aromatic material selected from the groupconsisting of:

wherein: R₁—R₁₀ each represent no substitution, mono-, di-, or tri-substitution, R₁₁—R₁₃, R₁₅—R₁₈, each represents no substitution, mono-,di-, or tri- substitution, and R₁₄, R₁₉, and R₂₀—R₂₅ each represents nosubstitution, mono-, di-, or tri- or tetra- substitution, and andwherein R₁—R₂₅ are the same or different substituents, and each isselected from the group consisting of alkyl, alkenyl, aryl, heteroalkyl,substituted aryl, substituted heteroaryl and heterocyclic groups. 46.The device of claim 45, wherein the aromatic material has a moleculardipole moment of less than about 2.0 debyes.
 47. The device of claim 46,wherein the aromatic material has a molecular dipole moment of zero. 48.The device of claim 45, wherein the aromatic material includes only Cand H atoms.
 49. A device, comprising: an anode; a cathode; a firstorganic layer disposed between the anode and the cathode, wherein thefirst organic layer comprises a material that produces phosphorescentemission when a voltage is applied between the anode and the cathode;and a second organic layer disposed between the first organic layer andthe cathode, wherein the second organic layer is in direct contact withthe first organic layer, and wherein the second organic layer comprisesan aromatic material wherein the aromatic material includes a corecomprising at least 3 phenyl rings, wherein each phenyl ring in the coreis connected to every other phenyl ring in the core either directly, orby a chain of phenyl rings that are either fused or attached by a singleC—C bond.
 50. The device of claim 49, wherein the aromatic materialincludes a core comprising at least 6 phenyl rings.
 51. A compound,having the structure:

wherein: R₁₁—R₁₃ each represents no substitution, mono-, di-, or tri-substitution, and wherein the substituents are the same or different,and each is selected from the group consisting of alkyl, alkenyl,alkynyl, aryl, heteroalkyl, and substituted aryl.
 52. The compound ofclaim 51, wherein R₁₁, R₁₂, and R₁₃ are each phenyl.
 53. The compound ofclaim 51, wherein the aromatic hydrocarbon material has the structure: